1. Field of the Invention
The present invention pertains to an electrical cable comprising at least one electrically insulating layer, and to a process for manufacturing such an electrical cable.
It applies typically but not exclusively to high-voltage (in particular 60 kV to 500 kV) electrical transmission cables and more particularly to cables intended for the transport of power as alternating current whose conductor is of the Milliken type. Conductors of the Milliken type are generally used for cables of large cross section, in particular greater than 1000 mm2.
More particularly, the invention relates to an electrical cable able to reduce the skin effect and to increase its capacity to transport current.
2. Description of Related Art
The skin effect is a phenomenon of electromagnetic origin, whereby the current has a tendency to flow only at the surface of conductors. This phenomenon exists for all conductors traversed by alternating currents and it causes the decay of the current density within the conductor on moving away from the periphery of the conductor.
With a view to reducing this effect, that is to say to increasing the surface area of the conductor that can receive a high current density, underground cables comprising a conductor of the Milliken type are traditionally used. They are generally composed of several elongate conducting elements, in particular of six elements, exhibiting a (cross) section in the shape of a sector of a circle and called segments or sectors. These sectors are mutually insulated and disposed so as to form a conductor of circular section. The skin effect is reduced because of the fact that each segment is insulated from its neighbor by a non-conducting material. Each sector generally consists of copper or aluminum metallic wires assembled together, in particular by stranding. The stranding of the individual metallic wires in each of the segments and the assembling of these same segments in a helix is well known under the expression “double cabling”. However, said double cabling entails alternately passing each of the metallic wires from the exterior of the conductor to the interior of the conductor. The voltage deep within the cable differs from that at the periphery of the cable. A transverse current between the metallic wires situated deep within the cable and those situated at the periphery of the cable is thereby created in each segment of the cable. The low contact resistance between the mutually adjacent metallic wires, which makes it impossible to halt the passage of the electric current from one metallic wire to another, results in a loss of effectiveness of this type of arrangement.
In order to remedy this problem, and thus to obtain a high contact resistance between the mutually adjacent conducting wires, a solution consists in covering each of the metallic wires with an electrically insulating layer of appropriate substance and appropriate thickness.
Accordingly, document U.S. Pat. No. 4,207,427 proposes to surround each of the metallic wires with an insulating coating, with a thickness of between 4 and 10 μm, composed of a cross-linked synthetic resin. This resin comprises a polyvinylacetalic resin cross-linked by means of at least one phenol aldehyde resin and of a melamine resin. The application of said coating to the metallic wire consists of a conventional enameling technique. The metallic wire is immersed in a bath containing said resin diluted in a liquid medium comprising xylene and the wire thus coated is subjected to a thermal treatment in a first zone of the oven at 300° C., and then in a second zone of the oven at a temperature between 400° C. and 500° C.
However, this type of coating exhibits the drawbacks of being very expensive and of having a considerably greater insulating capacity than needed (the dielectric rigidity obtained is about 3000 volts). Indeed, to obtain sufficient insulating capacity, it suffices to have a dielectric rigidity of at least 8 volts. Moreover, said coating is very difficult to remove when connecting the cable to one or more terminations on account of its overly strong adhesion to the conductor.
Furthermore, the process for preparing enameled metallic wires of the prior art exhibits a high production cost. Moreover, it can induce a partial annealing of the metallic wire since the baking cycle is performed at high temperatures, and generates toxic degradation products such as melamine and phenol. Finally, this process exhibits the disadvantage of using a liquid medium comprising xylene, which is difficult to evaporate.